The following relates to the lighting arts. It especially relates to flexible lighting strips for channel lettering, border lighting, and so forth. However, the following will also find application in conjunction with other lighting applications.
Light emitting devices, such as light emitting diodes, are suitable for use in lighting strips. For example, Southard et al., Int'l. Appl. Publ. No. WO 02/097770 A2 illustrates lighting strips including a flexible insulated cable with positive and negative conductors and modules bearing light emitting diodes. Each module includes insulation-displacing conductors that pierce the insulation and make electrical contact with the positive and negative conductors to provide electrical power to the module. By spacing the light emitting diode-bearing modules along the flexible insulated cable, a flexible lighting strip is formed.
Priddy et al., U.S. Pat. No. 6,505,956 illustrate lighting strips formed by daisy-chaining small light emitting diode-bearing printed circuit boards using flexible connecting conductors disposed between the printed circuit boards. Voltage-dividing resistors are included on each printed circuit so that the applied voltage can be larger than the forward voltage of the light emitting diodes. The difference between the applied voltage and the forward voltage of the light emitting diodes is accommodated by heat dissipation in the voltage-dividing resistors. The energy efficiency of such lighting strips is degraded by the power dissipation in the resistors.
Lin, U.S. Pat. No. 5,672,000 discloses a lighting strip including a flexible insulated cable with positive and negative conductors and a third series conductor, and modules bearing light emitting diodes that make electrical contact with the conductors of the insulated cable. A series-parallel lighting strip can be formed having a number of series portions in which each series portion includes a number of spaced apart modules. The first module of a series portion has insulation displacing conductors (IDC's) contacting the positive and series conductors; the next one or more modules have both IDC's connecting with the series conductor; and the last module in the series portion has IDC's contacting the series and negative conductors. The voltage applied between the positive and negative conductors drives the modules of each series portion electrically in series, so that the voltage across the series portion is the sum of the voltages across the modules in the series. Such series-parallel lighting strips can have a relatively high driving voltage and correspondingly lower driving electrical current, thus enabling a longer operable lighting strip length.
However, the lighting strip of Lin has certain disadvantages. The voltage across a given light emitting diode is controlled by the difference in applied driving voltage and by the voltage drops across each module of the series portion containing the given light emitting diode. These voltage drops, in turn, are affected by various factors which may vary with manufacturing variations and/or over time. For example, as the light emitting diodes heat up due to resistive heating during operation, the effective forward voltage increases due to a heat-induced increase in electrical resistance. If one of the modules fails, the remaining light emitting diodes will experience changed driving voltage.
More generally, existing lighting strips are sensitive to component variations. For example, in addition to the above-mentioned heating and light emitting diode failure issues, the present inventors have found that variability of forward voltage values in commercial lots of light emitting diodes is large enough that not all the light emitting diodes can be used in a parallel or series-parallel lighting strip such as that of Lin. Light emitting diodes at the high and low ends of the forward voltage range must be discarded, since their inclusion in a parallel or series portion or a series-parallel lighting strip would produce an unacceptable redistribution of voltage.
Another manufacturing issue with existing lighting strips is the number of different parts involved in lighting strip construction. Typically, the lighting strip includes light emitting devices, connectors, and two or more different types of insulation displacing conductors (IDC's). This multiplicity of different types of parts complicates manufacturing including the stocking of parts for the lighting strip.